1. Field of the Invention
The present invention relates to a unit-type linear displacement measuring apparatus having a detection head integrally formed with a frame body housing a main scale, wherein the detection head detects an amount of movement in a longitudinal direction on the basis of an amount of relative movement of an index scale that moves along the main scale, an end fixture therefor, and a fixing method using the same.
2. Description of the Related Art
In a machine tool, an industrial machine, or the like, an apparatus for detecting a linear displacement of objects to be measured (moving members) that move relatively to each other is used for such as control. As such a linear displacement detecting apparatus, a unit-type linear scale is generally used in which an elongated frame body housing a main scale and a detection head housing a detecting portion such as an index scale capable of relatively moving with respect to the frame body are formed integrally.
In the unit-type linear scale, the main scale is generally incorporated in the frame body made of aluminum. The frame body 10 is directly fixed mainly to a stage-sliding portion of the machine tool, the industrial machine, or the like, with screws 12 at a plurality of positions, as shown in FIG. 6 (this type is referred to as a multi-position fixed type). Alternatively, the frame body 10 is fixed to the stage-sliding portion with the screws 12 mainly at both ends thereof via fixing blocks 14 for fixation, as shown in FIG. 7 (this type is referred to as a double-end fixed type).
However, the frame body 10 is made of aluminum, whereas a mating member to which the frame body is fixed is mainly made of iron, so that the following problem occurs due to the change in an ambient temperature.
Namely, as a generality, a coefficient of linear expansion of iron, of which the mating member is made, is 11xc3x9710xe2x88x926, whereas a coefficient of linear expansion of aluminum, of which the frame body 10 is made, is 23xc3x9710xe2x88x926 In the state in which aluminum is fixed to iron, the elongation of aluminum is restricted due to the difference in the coefficient of linear expansion, so that a thermal stress occurs. There is a possibility that deflection and torsion occur in the frame body owing to the thermal stress, and accuracy deteriorates as the main scale in the frame body conforms thereto.
In particular, with the double-end fixed type shown in FIG. 7, the structure is such that deflection or torsion is likely to occur in the frame body 10, so that the aforementioned tendency occurs noticeably.
FIG. 8 shows a fixing mechanism of the double-end fixed type shown in FIG. 7 and an example of the behavior the deflection of the frame body in the case where the ambient temperature has arisen. In FIG. 8, the frame body 10 and the fixing blocks 14 are completely joined with screws 16, and the fixing blocks 14 are fixed to a mating surface (iron) 8 with the screws 12 (see FIG. 7).
When the ambient temperature rises in this state, the frame body 10 tends to be elongated in the left-and-right direction in the drawing (this direction is set as a Z direction), and the amount of its elongation is greater than that of the mating surface 8. At this time, the frame body 10 is elongated relatively easily at the position in an upward direction of the drawing (this direction is set as a +Y direction) in the cross section of the frame body 10. However, the elongation of the frame body 10 in the downward direction of the drawing (this direction is set as a xe2x88x92Y direction) is restricted by the fixing blocks 14, and a thermal stress occurs. Furthermore, since the thermal stress is not a force uniformly distributed in the Y direction of the cross section, this thermal stress is converted into bending moment with respect to the cross section, so that the frame body 10 is curved in the +Y direction. When the ambient temperature has conversely dropped, the frame body 10 is curved in the xe2x88x92Y direction for the same reason.
When the frame body 10 is thus curved, the main scale inside it is also curved, resulting in the deterioration of accuracy. When the thermal stress exceeds the fastening forte with which the frame body 10 and the fixing blocks 14 are fastened, an offset in the joint occurs, so that the relative positional relationship with the mating member 8 changes. This constituted a factor of uncertainty of the origin, and caused the problem that the reproducibility of measured values with respect to the temperature deteriorates.
As a mechanism for absorbing such elongation of the frame body, JP-A-8-14819 discloses a technique in which an end fixture 18, such as the one shown in FIG. 9, is provided at each end. In this end fixture 18, important dimensions for determining the rigidity in the Z direction are t and L. It is necessary to make the dimension of L small in compliance with the demand in recent years for the compact size, and in order to reduce the occurrence of the thermal stress in that state, it is also necessary to make t small. In this case, it is impossible to obtain sufficient rigidity capable of withstanding external forces such as vibrations.
The invention has been devised to overcome the above-described conventional problems. It is an object to provide a unit-type linear displacement measuring apparatus capable of reducing an occurrence of a thermal stress by positively absorbing an elongation of a frame body due to a change in the ambient temperature while ensuring rigidity, an end fixture therefor, and a fixing method using the same.
To attain the above object, in accordance with the present invention, there is provided a unit-type linear displacement measuring apparatus comprising: a frame body extending in a length measuring direction and housing a main scale; and a first end fixture fixed to one end in the length measuring direction of the frame body and having a parallel plate spring mechanism capable of absorbing a thermal expansion of the frame body, wherein the frame body has a plate spring mechanism which is provided at the one end of the frame body fixed by the first end fixture and which is capable of absorbing a deflection of the one end of the frame body in a direction perpendicular to a direction to which the thermal expansion occurs.
The above-described unit-type linear displacement measuring apparatus preferably further comprises: a second end fixture fixed to another end in the length measuring direction of the frame body and having a parallel plate spring mechanism capable of absorbing the thermal expansion of the frame body.
In addition, the above-described unit-type linear displacement measuring apparatus preferably further comprises: an intermediate-portion fixing block fixed to a substantially central portion in the length measuring direction of the frame body and having rigidity higher than that of the first end fixture.
In addition, to attain the above object, there is provided an end fixture for a unit-type linear displacement measuring apparatus for being fixed to one end in a length measuring direction of a frame body which houses a main scale and which extends in the length measuring direction, so as to fix the frame body to an object to be measured, comprising: a parallel plate spring mechanism capable of absorbing the thermal expansion of the frame body.
Furthermore, to attain the above object, there is provided a method of fixing a unit-type linear displacement measuring apparatus comprising the step of: fixing an object to be measured to a frame body which extends in a length measuring direction and which houses a main scale with a first end fixture fixed to one end in the length measuring direction of the frame body and having a parallel plate spring mechanism capable of absorbing the thermal expansion of the frame body.
The above-described method of fixing a unit-type linear displacement measuring apparatus preferably further comprises the step of: fixing the object to be measured to the frame body with a second end fixture fixed to another end in the length measuring direction of the frame body and having a parallel plate spring mechanism capable of absorbing the thermal expansion of the frame body.
The above-described method of fixing a unit-type linear displacement measuring apparatus preferably further comprises the step of: fixing the object to be measured to the frame body with an intermediate-portion fixing block fixed to a substantially central portion in the length measuring direction of the frame body and having rigidity higher than that of the first end fixture.